The present invention relates generally to operational amplifiers operable from low supply voltages, and especially to class AB output stages which are operable from supply voltages as low as 1 volt.
Various known circuits for CMOS amplifier output stages are operable from fairly low supply voltages, i.e., as low as roughly 2 volts, and also are operable over a relatively large supply voltage range. The relevant prior art is believed to include U.S. Pat. No. 6,657,495 “Operational Amplifier Output Stage and Method” issued Dec. 2, 2003 to Ivanov et al. and U.S. Pat. No. 7,088,182 “Class AB Output Stage Circuit with Stable Quiescent Current” issued Aug. 8, 2006 to Ivanov.
“Prior Art” FIG. 1 herein shows an operational amplifier 1 including a rail-to-rail differential input stage 2 which feeds into a folded cascode stage 3. Folded cascode stage 3 supplies a current IIN1 through conductor 11 to one input of a class AB output stage 4. Folded cascode stage 3 also sinks a current IIN2 through conductor 12 from the other input of class AB output stage 4. The currents IIN1 and IIN2 are dependent on the differential input voltage Vin+−Vin−. Typically, the magnitude of the threshold voltage VTP of P-channel transistors is slightly greater than the magnitude of the threshold voltage VTN of N-channel transistors in a CMOS integrated circuit. Therefore, for a typical CMOS integrated circuit, the minimum supply voltage VDD at which operational amplifier 1 of FIG. 1 is operable is equal to the sum of the voltage drops across P-channel transistors 4F and 4G and the voltage drop across current source 4H, which at normal temperatures is in the range from roughly 2.2 volts to 2.5 volts, assuming the lower supply voltage VSS is at ground. Furthermore, it is believed that no other prior art class AB output stages are capable of operating at voltages less than approximately 1.5 volts.
There is increasing demand for low-cost, low-power CMOS operational amplifiers which are operable from supply voltages as low as approximately 1 volt and which also are operable over a supply voltage range of at least about 0.9 to 5.0 volts in order to allow power to be supplied by various common batteries. However, no satisfactory solutions to this need have been found in the available literature. All of the closest prior art very-low-voltage class AB output stages are characterized by poor linearity, limitation of the output current, poor stability of the class AB current, and/or unacceptable complexity. It would be highly desirable to have a class AB output stage for a CMOS amplifier operating at a supply voltage as low as 1.0 volts or less using current state-of-the-art CMOS manufacturing processes.
Thus, there is an unmet need for a CMOS class AB output stage that is operable at power supply voltages at least as low as approximately 0.9 to 1.0 volts.
There also is an unmet need for a CMOS class AB output stage that is operable at power supply voltages as low as approximately 0.9 to 1.0 volts and also is operable at a power supply voltage as high as approximately 5 volts.
There also is an unmet need for a CMOS class AB output stage that is operable at power supply voltages as low as approximately 0.9 to 1.0 volts and which is not characterized by poor linearity and/or limitations of the amount of output current and/or poor stability of the output current.
There also is an unmet need for a CMOS class AB output stage that is operable at power supply voltages as low as approximately 0.9 to 1.0 volts and which has a simple circuit configuration less complex than the closest prior art low voltage class AB output stages.